If high speed isn't working, make sure you have power to the high speed relay. High is direct feed, and doesn't go through the resistor pack.After 3 years with no heat, I finally made it work again! Well, mostly. High doesn't work, but the other 3 speeds do. Weird.
New blower motor, resistor (old looked fine, but might as well replace it), new heater core awhile back, vacuum ball doohickey.